This invention relates to an organic light emitting diode (OLED) electroluminescent (EL) device comprising a light emitting layer (LEL) containing a light emitting material that contains an organometallic complex comprising Pt or Pd metal and a tridentate (N^N^C) ligand, wherein the tridentate (N^N^C) ligand represents a ligand that coordinates to the metal through a nitrogen donor bond, a second nitrogen donor bond, and a carbon-metal bond, in that order.
While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, xe2x80x9cDouble Injection Electroluminescence in Anthracenexe2x80x9d, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 xcexcm). Consequently, operating voltages were very high, often  greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g.,  less than 1.0 xcexcm) between the anode and the cathode. Herein, the term xe2x80x9corganic EL elementxe2x80x9d encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al (J. Applied Physics 1989, 65, 361 0). The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited siniglet state is created when excitons formed in an OLED device transfer their energy to the excited state of the dopant. However, it is generally believed that only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplets, which cannot readily transfer their energy to the singlet-excited state of a dopant. A large loss in device performance efficiency results since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their excited state energy to a dopant, if the dopant molecule""s triplet excited state is sufficiently lower in energy. If the triplet state of the dopant is emissive, it can produce light by phosphorescence, wherein phosphorescence is a luminescence involving a change of spin state between the excited state and the ground state. In many cases singlet excitons can also transfer their energy to lowest singlet excited state of the same dopant molecule. The singlet excited state can often relax, by an intersystem crossing process, to produce the emissive triplet excited state. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission.
One class of usefull phosphorescent materials is transition metal complexes having a triplet-excited state. For example, fac-tris(2-phenylpyridinato-N,C2xe2x80x2)iridium (III)(Ir(ppy)3) strongly emits green light from a triplet excited state owing to the large spin-orbit coupling of the heavy atom and to the properties of the lowest excited state, which is a charge transfer state having a Laporte-allowed (orbital symmetry) transition to the ground state (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc. 1985, 107, 1431; M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Gudel, M. Fortsch, and H.-B. Burgi, Inorg. Chem. 1994, 33, 545. Small-molecule, vacuum-deposited OLEDs having high efficiency have also been demonstrated with Ir(ppy)3 as the phosphorescent material and 4,4xe2x80x2-N,Nxe2x80x2-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett. 1999 4, 75, T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys. 1999, 38, L1502).
Another class of phosphorescent materials include compounds having interactions between atoms having d10 electron configuration, such as Au2(dppm)Cl2 (dppm=bis(diphenylphosphino)methane) (Y. Ma et al, Appl Phys. Lett. 1998, 74, 1361). Still other examples of useful phosphorescent materials include coordination complexes of the trivalent lanthanides such as Tb3+ and Eu3+(J. Kido et al, Appl. Phys. Lett. 1994, 65, 2124). While these latter phosphorescent compounds do not necessarily have triplet states as the lowest excited states, their optical transitions do involve a change in spin state of 1 and thereby can harvest the triplet excitons in OLED devices.
Although many phosphorescent Ir complexes have been described as usefull in an EL device, Pt-based organometallic complexes have not been examined as extensively. Some Pt-based phosphorescent complexes include cyclometallated Pt (II) complexes, such as cis-bis(2-phenylpyridinato-N,C2xe2x80x2)platinum (II), cis-bis(2-(2xe2x80x2-thienyl)pyridinato-N,C3xe2x80x2)platinum (II), cis-bis(2-(2xe2x80x2-thienyl)quinotinato-N,C5xe2x80x2)platinum(II), (2-(4,6-difluorophenyl)pyridinato-N,C2xe2x80x2)platinum (II) acetylacetonate, or (2-phenylpyridinato-N,C2xe2x80x2)platinum (II) acetylacetonate. Pt (II) porphyrin complexes such as 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphineplatinum (II) are reported in U.S. Pat. No. 6,048,630 as useful phosphorescent materials in an electroluminescent device, although they did not give a very high luminance yield. Recently, C.-M. Che, W. Lu, and M. Chan reported organometaflic light-emitting materials and devices based on (N^N^C) tridentate-cyclometalated Pt (II) acetylides (U.S. 2002/0179885 A1 and references cited therein).
It is a problem to be solved to provide new organometallic compounds that will function as phosphorescent materials having useful light emissions.
The invention provides an electroluminescent device comprising a cathode, an anode, and, located there between, a light emitting layer (LEL) containing a light emitting material that contains an organometallic complex comprising Pt or Pd metal and a tridentate (N^N^C) ligand, wherein the tridentate (N^N^C) ligand represents a ligand that coordinates to the metal through a nitrogen donor bond, a second nitrogen donor bond, and a carbon-metal bond, in that order, wherein at least one of the nitrogen donors is part of an aromatic ring or an imine group, and wherein the Pt or Pd atom also forms a bond to an anionic ligand group L, wherein L represents alkyl, alkenyl, aryl, or a cyano carbon, or halogen, or RX, wherein X represents a substituent that forms a bond to the Pt or Pd atom and wherein X represents N, O, S, or Se, and R represents a substituent.